Multi-plate clutches are often applied to vehicle drivetrains for the purpose of controlling torque transmission, or limiting differential motions. While lubricant oil is filled in devices constituting a vehicle drivetrain, generally this oil is further used to operate the multi-plate clutches in a mode of a wet type. While the multi-plate clutches create sufficiently large braking force if sufficient connection force is applied thereto, moderate reduction in the connection force, according to its degree, leads to moderate reduction of the braking force as the lubricant oil promotes slippage among the clutch plates, thereby enabling continuous control of the torque transmission or control of the degree of the differential motion limitation.
Because the slippage among the clutch plates frequently occurs, although the lubricant oil intervenes, the respective clutch plates unavoidably suffer abrasion. Wear particles will spread over and therefore degrade the lubricant oil. Further the wear particles suspended in the lubricant oil can accumulate in other constituents of the drivetrain and then give rise to drag on its operation.
To prevent clutches from overheating, or for any other purpose, generally, one group of the plates of a multi-plate clutch contains friction members adhered thereon. Generally available are paper friction members formed of pulp or synthetic fibers; vacant pores among fibers thereof contribute to preservation of the lubricant oil to maintain its lubrication property. Further, carbon powder, as an antifriction material, is often made to be supported in these pores. Japanese Patent Application Laid-open No. H11-201183 discloses a related art.